The present invention relates to a method for measuring an acoustic transmission characteristic between sound sources and a predetermined listening point such as inside a vehicle.
In order to measure the acoustic transmission characteristic in a given sound field, heretofore a method has generally been employed whereby a dummy head on the ears of which microphones are arranged is set at the listening point, and the characteristic is determined from the outputs of the microphones.
However, this measuring method is disadvantages in that when a real person is present in the sound field, the sound field is disturbed by the sound absorption of the person's clothes and body, and this effect cannot be measured. Especially in the case of a restricted sound field such as that inside a vehicle, the sound field characteristic when no person is present in the sound field is greatly different from the sound field characteristic provided when a person is present. Moreover, with this method, it is rather difficult to integrally evaluate a sound field including a plurality of sound sources. Also, integrated measurement results including various effects of the ears, such as directionality and the addition effect, cannot be obtained using this method.
Moreover, since the dummy head has a directional pattern which changes intricately with frequency and the dummy head is comparable in size with the wavelengths of the audio frequency sounds, and the dummy head has both right and left ears, it is difficult to accurately evaluate a sound field according to the conventional method in a case where the sound sources are sequentially driven.
The invention further relates to automatic graphic equalizers, and more particularly to an automatic graphic equalizer in which a test signal is used to measure a frequency characteristic in a sound field, and the test result is utilized to automatically control the graphic equalizer.
A conventional device of this type is constructed as shown in FIG. 1. In FIG. 1, right and left channel audio signals together with a "pink" noise signal from a pink noise generating source 3-8 are applied to a signal applying and selecting switch 3-1. The two audio signals are subjected to frequency characteristic control by a graphic equalizer 3-2, and then applied through amplifiers 3-3R and 3-3L to loudspeakers 3-4R and 3-4L, respectively, thus producing sound radiated inside a vehicle, for instance.
The sound thus radiated is received by a microphone 3-5, and the output signal of the microphone 3-5 is applied to a frequency characteristic measuring section 3-6 where the signal is divided into frequency bands. Level detection is carried out for every band, as a result of which d.c. data values corresponding to the signal powers in the various bands are obtained. These data values are converted into digital data, which is sequentially stored in a controller 3-7, implemented with a microcomputer. The band characteristics of the graphic equalizer 3-2 are automatically controlled according to the digital data thus stored.
FIG. 2 shows an example of the frequency characteristic measuring section 3-6. The output signal from the microphone is applied through a microphone amplifier 3-9 to a frequency-band dividing BPF (bandpass filter) 3-10. The outputs of the BPF 3-10 are converted into d.c. signals by a rectifying circuit 3-11 and a smoothing circuit 3-12. One of the d.c. signals at a time is selected by a switch 3-13 and applied to an A/D (analog-to-digital) converter 3-14.
It is assumed that the above-described circuit provides a frequency characteristic as shown in FIG. 3A. In this case, the graphic equalizer 3-2 is adjusted so that the sound field frequency characteristic is flat, whereupon data values D.sub.1, D.sub.2, ... and D.sub.n (n=5 in FIG. 3A) are obtained. For this purpose, the graphic equalizer 3-2 should be adjusted so as to have a characteristic as shown in FIG. 3B.
In order to obtain adjustment data values G.sub.n (n=1 through 5) as shown in FIG. 3B, the following calculations are carried out by the controller 3-7. ##EQU1## D.sub.n =D.sub.n -D.sub.av, and G.sub.n =-.DELTA.D.sub.n (n=1 through 5).
The frequency characteristic in the sound field is made substantially flat by controlling the graphic equalizer 3-2 with the adjustment data G.sub.n. Under this condition, the application of the pink noise is suspended and the switch 3-1 is operated to apply the audio signals. However, instead of this method, the application of the pink noise may be continued, the frequency characteristic measurement carried out again, and according to the measurement result, adjustment performed again. In order to take maximum advantage of the dynamic range of the A/D converter, the microphone amplifier 3-9 is designed so that the amplifier gain is controlled by the controller 3-7. However, it may be replaced by an amplifier having an ALC (automatic level control) function.
In the above-described conventional device, the measuring microphone 3-5 is built into the system, and the number of microphones is limited to only a single one. Therefore, the device often suffers from the difficulty that the frequency characteristic at the listening point does not coincide with that at the position of the microphone. In the case where the measurement is carried out with one microphone set at the listening point, the acoustic waves travel along paths 3-41 and 3-42 to reach the microphone, as shown in FIG. 4A. However, since the person listens to the sound with both ears and there are paths 3-43 and 3-44 in FIG. 4B in addition to the aforementioned paths 3-41 and 3-42, the person hears the sounds as the sum of these waves. Accordingly, the frequency characteristic measured with only one microphone does not match well with the actual hearing frequency characteristics.
Still further, the invention relates to an automatic sound field correcting system for automatically correcting a frequency characteristic in a listening space as desired.
An example of a conventional automatic sound field correcting system is shown in FIG. 5. In this system, an audio signal outputted by a signal source 4-1 such as a tape deck is supplied to one input of an adder 4-2, to the other input of which the output of a test signal generator 4-3 is connected. The test signal generator 4-3 generates a test signal, such as a "pink" noise signal, having a known characteristic. The output terminal of the adder 4-2 is connected to a graphic equalizer 4-4 having a plurality of adjustment bands. The output terminal of the graphic equalizer 4-4 is the output terminal of the system. The output terminal is connected to a power amplifier 4-5 in an acoustic reproduction device. The power amplifier 4-5 drives a loudspeaker 4-6.
Sound ratified from the loudspeaker 4-6 is detected by a microphone 4-7, the output terminal of which is connected to a frequency characteristic detecting circuit 4-8. The circuit 4-8 has a plurality of BPFs (bandpass filters) having different passbands for detecting the frequency characteristic in the sound field according to the output signal of the microphone 4-7. The frequency characteristic detecting circuit 4-8 is connected through an A/D (analog-to-digital) converter 4-9 to a control circuit 4-10 The control circuit 4-10, implemented with a microcomputer, controls the operation of the test signal generator 4-3 and the gain levels of the adjustment bands of the graphic equalizer 4-4.
In this circuit, in response to an automatic sound field correcting instruction from a keyboard (not shown) or the like, the control circuit 4-10 causes the test signal generator 4-3 to operate. As a result, the test signal is applied through the adder 4-2 to the graphic equalizer 4-4. In this operation, the application of the audio signal to the adder 4-2 from the signal source 4-1 is suspended, and the output frequency characteristic of the graphic equalizer is made flat with the gain levels of the adjustment bands being made equal. The test signal outputted by the graphic equalizer is applied through the power amplifier 4-5 to the loudspeaker 4-6, as a result of which sound corresponding to the test signal is radiated in the listening space from the loudspeaker 4-6. At the listening point, the microphone 4-7 detects the sound pressure of the test signal reproduced sound to provide a sound pressure signal, which is supplied to the frequency characteristic detecting circuit 4-8. The circuit 3-8 detects the sound pressure signal level according to the adjustment bands of the graphic equalizer 4-4, and the resultant detection levels are converted into digital data by the A/D converter 4-9. The digital data is applied to the control circuit 4-10. The control circuit 4-10 detects the frequency characteristic of the detected sound pressure signal, i.e., the frequency characteristic in the sound field according to the digital data, controls the gain levels of the adjustment bands of the graphic equalizer 4-4 so that the frequency characteristic in the sound field becomes flat, and then stops the operation of the test signal generator 4-3. Thus, automatic sound field correcting operation has been accomplished.
In general, when a person listens to reproduced sound in a listening space, the effects of the head, the face, the ears, etc. cannot be neglected. However, if the listening space, the signal source, and the power amplifier, the loudspeaker, etc., have no element which may disturb the sound field frequency characteristic, it can be assumed that the sound field frequency characteristic is relatively flat. On the other hand, it is obvious that it is better to perform sound field correction under the practical condition that a person is present in the listening space. When automatic sound field correction is carried out in an ideal listening space in the same manner with the microphones of the microphone unit 4-7 set at the inlets of the external auditory canals as shown in FIG. 6, the sound field correction is affected by the head, the faces, the ears, etc. As a result, the sound field frequency characteristic obtained from the output signal of the frequency characteristic detecting circuit is different from that which is obtained in the case where a single microphone is used, for instance, the resultant frequency characteristic may not be flat, being irregular especially in the high frequency range, as indicated by the solid line a in FIG. 7. If the graphic equalizer 4-4 is adjusted so that its frequency characteristic is opposite to the frequency characteristic indicated by the solid line a, theoretically the sound field frequency characteristic is made flat. However, this method suffers from the problem that the sound produced according to the method produces a slightly unnatural auditory sensation.
Yet moreover, the invention relates to acoustic characteristic measuring device for measuring the acoustic characteristic between a plurality of sound sources provided in correspondence to a plurality of channel signals and a listening point at which a person listens to reproduced sounds from these sound sources.
A conventional acoustic characteristic measuring device of this type is as shown in FIG. 8.
As shown in FIG. 8, a pair of loudspeakers 5-1R and 5-1L are provided at the right and left sides in a room A. A signal outputted by a signal generator 5-2 is applied through a changeover switch 5-3 to the loudspeaker 5-1R or 5-1L. The changeover switch 5-3 is operated so that the signal is applied through a transmission characteristic correcting device 5-4L such as a graphic equalizer and an amplifier 5-5L to the left loudspeaker 5-1L, or it is applied through a transmission characteristic correcting device 5-4R and an amplifier 5-4R to the right loudspeaker 5-1R. The signal outputted by the signal generator 5-2 may be a pink noise signal, impulse signal, white noise signal, warble tone signal, or sine wave signal.
In order to measure the transfer function at the sound field formed by sound reproduced by the loudspeakers 5-1R and 5-1L, or in order to electrically correct the sound field characteristic according to the measurement results, a nondirectional microphone 5-6 is set at a certain point (such as a listening point where a person listens to sound from the loudspeakers 5-1R and 5-1L) in the sound field. The output of the microphone 5-6 is displayed on a display unit 5-7 to measure the sound pressure versus frequency characteristic.
A stereo acoustic reproduction system of more than one channel uses more than one loudspeaker, and there are generally available two acoustic characteristic measuring methods. In one of the methods, the measurement is carried out by applying the same phase signal to more than one signal source, and in the other method, the measurement is performed for each of the signal sources. In the former method, signals from the loudspeakers are subjected to vector addition at the position of the microphone, and the result of the vector addition is outputted as a sound pressure signal received by the microphone. In the latter method, the signals are outputted as separate sound pressure signals which the microphone receives from the two separate loudspeakers.
In the above-described conventional device, a single-point measurement is carried out in which only one microphone is installed in the sound field in which no person is present. However, of course there is a person present in an actual sound field. When a person is present in the sound field, the sound field is disturbed by the person's body. Moreover, the directional pattern of human ears can differ greatly from person to person. Therefore, it is difficult to perform measurements which yield optimum listening conditions for all persons. In the case of stereo reproduction, it is difficult to measure a synthetic characteristic. Furthermore, in the case where, as in a mobile acoustic device, the listening point is symmetric in position, it is considerably difficult generally to evaluate the sound field with two transfer functions in the paths from the loudspeakers to the listening point converted into one.